Impedance matching ferrite transformer wherein center conductor of associated coaxial cable forms primary winding



United States Patent IMPEDANCE MATCHING FERRITE TRANS- FORMER WHEREIN CENTER CONDUC- TOR 01F ASSOCIATED COAXIAL CABLE FORMS PRIMARY WINDING John G. Campbell, R0. Box 53, Mineral Wells, Tex. 76067 Filed Sept. 14, 1962, Ser. No. 223,736 4 Claims. (Cl. 333 -33) My invention relates to electrical power transfer devices and more particularly to impedance matching devices. Whenever, in an electrical system, the impedance of the source and the load are fixed and unequal in magnitude, a transformer may be inserted between them either to step-up or step-down the impedance of the load for a better transfer of power. Such an unbalanced relation is found oftentimes in the antenna transmission line of the ordinary household-type television receiver which employs a coaxial cable and plug to receive an input signal from a community-type antenna. In such an installation, a power transfer device like an impedance matching transformer, or balun, may be inserted in the antenna transmission circuit to match the line impedance to the receiver set impedance.

A typical impedance matching device, such as is found in the prior art, includes one balun and two isolation capacitors. The balun and the capacitors are assembled as one unit by making at least four solder connections and then encapsulating them in an insulating sheath. It is recognized that the cost of labor and materials to make and assemble such prior art devices is a substantial proportion of the cost of the item. It is also recognized that a significant reduction in cost may be achieved by reducing the complexity of the transformer structure.

Accordingly, it is the general object of the present invention to provide an improved impedance matching device.

Another object of the invention is to provide an improved impedance matching device which has a minimum of parts and is inexpensive to make.

Another object of the invention is to provide an impedance matching device to be interposed between the input terminals of a television receiver and a coaxial transmission line, which device is characterized by simplicity and economy.

Another object of the invention is to provide an improved impedance matching device which is compact and rugged.

An important feature of the present invention resides in the concept of an extremely simple impedance match ing device wherein the center conductor of a coaxial transmission line is utilized as the primary winding of an impedance matching transformer, it being simply wrapped around the transformer core and then connected to the cable outer conductor or sheath. Another feature of the invention is the concept of utilizing a piece of standard 300 ohm lead-in cable as the secondary winding of the impedance matching device. Still another feature is the concept of encapsulating the impedance matching device in a length of shrink tubing.

For a further understanding of the invention and further objects, features, and advantages thereof, reference may now be had to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a fragmentary schematic perspective view showing a coaxial transmission line terminating in an impedance matching device of the present invention which is in turn connected to the input terminals of a television receiver;

FIG. 2 is an electrical schematic diagram of the transmission line and impedance matching device of FIG. 1;

3,327,253 Patented June 20, 1967 FIG. 3 is a schematic elevational view, partly broken away, of an impedance matching device in accordance with a preferred embodiment of the invention;

FIG. 4 is a sectional view taken along line 1VIV of FIG. 1; and

FIG. 5 is a plan view of only the impedance matching device core.

Referring now to the drawings, in FIG. 1 there is shown a portion of the back of a conventional television receiver 11 having a terminal plate 17 to which the receiver antenna signal input leads (not shown) are connected. A coaxial cable 13 terminates at one end in an impedance matching device 21 of the present invention and at the other end in a conventional plug 19. The plug 19 is adapted for insertion into a conventional jack 20 which is mounted to a wall receptacle plate 15. The input terminals 22 of the jack 20 are connected to a television antenna signal coaxial transmission line (not shown). The impedance matching device 21 is provided output terminals 41 which are connected to the antenna signal input terminals of the television receiver.

FIGS. 3 and 4 show an impedance matching device 21 in accordance with a preferred embodiment of my invention. The impedance matching device 21 is in the form of a transformer having a core 23, a primary wind ing 25, a secondary winding 27, and a sealing cover or housing 29. The core 23 is a block of ferrite which is provided with a T-shaped slot or groove 31 in its perimetrical portion for receiving the transformer windings 25, 27.

The primary winding 25 comprises a single turn of the center conductor 33 of the coaxial cable 13- which turn is disposed, preferably, in the center portion of the head of the T-shaped groove 31. The free end 35 of the center conductor 33 is connected to the coaxial cable sheath 37 by a cable crimping device 39. If desired, the free end may be soldered to the cable sheath, or it may be connected in any other suitable manner.

The secondary winding 27 comprises two turns of an insulated conductor which is preferably disposed in the T-shaped groove 31 so that there is one turn on each side of the primary turn, as illustrated in FIG. 4. The free ends of the secondary winding 27 extend from the side of the core 23 opposite to the cable 13 and are provided with conventional spade terminals 41. Other types of terminals may, of course, be effectively employed. If desired, the secondary winding could be simply the conductors of a piece of standard 300 ohm television lead-in cable. One conductor of such cable could be wrapped around the core and then connected to the other, or both conductors could be wrapped around the core and then connected. It is desirable to encapsulate the transformer assembly by placing around it an appropriate length of material known in the art as shrink tubing 29. As is Well known, the shrink tubing, when heated, shrinks considerably and forms a tight-fitting cover or housing around the assembly. The assembly is, in effect, encapsulated in a substantially impervious and insulated envelope which also improves the structural rigidity of the unit. If desired, the core assembly may be wrapped with foil or other shielding material before the shrink tubing is applied. The shielding material would, of course, be in contact with the coaxial cable sheath.

While in a preferred embodiment of the invention the core 23 is made of ferrite, it is acknowledged that other suitable materials cay be found which would be used with equivalent effectiveness. Moreover, it is apparent that the core 23 may be produced in other suitable forms than the grooved elongated oval bobbin form shown and described hereinbefore. For example, one such other form might be simply a pair of sleeves; While in another form, it could 3 be a block with a pair of parallel passages through the block.

While the device has been shown in FIG. 1 as applied in a television receiver installation, it is to be clearly understood that it is not restricted solely to such application.

It should be apparent from the foregoing that the impedance matching device of the present invention is extremely simple, requiring no special parts other than a core, a piece of shrink tubing, and in some cases a crimp ring. The device requires little labor in manufacture, with a maximum of only two solder connections to be made, and in some cases none at all. The device is also compact, rugged, and etfective.

While I have shown my invention in only one form, it will be obvious to those skilled in the art that it is not so limited, but is susceptible of various changes and modifications without departing from the spirit thereof.

I claim:

1. An impedance matching device comprising: a length of coaxial cable having a center conductor and an outer conductor; a magnetic core of ferrite material having a peripheral groove therein; a primary winding surrounding said core and disposed in said groove, said primary winding being a loop of the center conductor of said coaxial cable with the free end of said center conductor electrically connected to the outer conductor of said cable; a secondary winding on said core and disposed in said groove with a pair of terminal ends projecting therefrom the number of turns of said secondary winding on said core being N times greater than the number of loops of said center conductor comprising the primary winding where N is approximately the square root of the ratio of the impedance connected to the primary winding to the impedance connected to the secondary winding; and means encapsulating said device.

2. An impedance matching device for use in the signal input circuit of a television receiver comprising: a length of coaxial cable having a center conductor and an outer conductor for connection at one end to a signal source; a magnetic core of ferrite material; a primary winding on said core made up of a loop of said center conductor at the other end of said cable, with the free end of said center conductor electrically connected to said outer conductor; and a secondary winding of two loops on said core made up of the conductors at one end of a length of 4 300 ohm television lead-in cable, said last mentioned conductors being electrically joined at their end portions.

3. An impedance matching device comprising: a length of coaxial cable having a center conductor and an outer conductor; a magnetic core of ferrite material; a primary win-ding on said core made up of at least one loop of said center conductor with the free end of said center conductor electrically connected to said outer conductor; and a secondary winding on said core, the number of turns of said secondary winding on said core being N times greater than the number of loops of said center conductor comprising the primary winding where N is approximately the square root of the ratio of the impedance connected to the primary winding to the inpedance connected to the secondary winding.

4. An impedance matching device comprising: a length of coaxial cable having a center conductor and an outer conductor, a magnetic core comprising a body of ferrite material having a pair of substantially parallel passages formed therein, a primary winding on said core comprising a single loop of said center conductor positioned in said pair of passages with the free end of said center conductor electrically connected to said outer conductor; and a secondary winding on said core comprising a double loop of an insulated conductor positioned in said pair of passages whereby impedance matching is obtained between an impedance connected to the secondary winding four times as great as the imepdance connected to the primary winding.

References Cited UNITED STATES PATENTS 2,189,309 2/1940 Carlson 343851 2,611,080 9/1952 Brough 333--78 2,613,268 10/1952 Jaspers 33378 2,628,312 2/1953 Peterson et al 343-851 2,666,183 1/1954- Ocnascbeck 333-- 81 2,679,007 5/1954 Sands 33378 2,721,260 10/1955 Schmidt 325381 2,838,737 6/ 1958 Duncan 336- 2,855,571 10/1958 Kleespies 33324 3,056,925 10/1962 Borck et a1. 345862 HERMAN KARL SAALBACH, Primary Examiner.

C. BARAFF, Assistant Examiner. 

1. AN IMPEDANCE MATCHING DEVICE COMPRISING: A LENGTH OF COAXIAL CABLE HAVING A CENTER CONDUCTOR AND AN OUTER CONDUCTOR; A MAGNETIC CORE OF FERRITE MATERIAL HAVING A PERIPHERAL GROOVE THEREIN; A PRIMARY WINDING SURROUNDING SAID CORE AND DISPOSED IN SAID GROOVE, SAID PRIMARY WINDING BEING A LOOP OF THE CENTER CONDUCTOR OF SAID COAXIAL CABLE WITH THE FREE END OF SAID CENTER CONDUCTOR ELECTRICALLY CONNECTED TO THE OUTER CONDUCTOR OF SAID CABLE; A SECONDARY WINDING ON SAID CORE AND DISPOSED IN SAID GROOVE WITH A PAIR OF TERMINAL ENDS PRO- 